Revert of Add constant color GrFP. (patchset #10 id:180001 of https://codereview.chromium.org/978713002/)

Reason for revert:
Revert while investigating assertions.

Original issue's description:
> Add constant color GrFP.
>
> Committed: https://skia.googlesource.com/skia/+/dfbbec436cbcacc3270d4b28357c8393e67d6494

TBR=egdaniel@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1055023002
8 files changed